Control devices of this kind are used to control the performance or to regulate a predeterminable speed of the hydrostatic gear depending on the speed of the driving motor. Preferred applications are devices and machines involving a robust, powerful and relatively small working drive, particularly construction vehicles such as cranes, winches, excavators, bulldozers, wheel loaders or the like where it is usual to use an existing driving unit for the travelling drive as well.
A control device of the above-mentioned kind is described and illustrated in German patent specification 30 41 856. This known construction is designed for reversible operation. Apart from this it exhibits the following disadvantage. When, in operation without any change in the delivery direction of the main pump or in the direction of travel, a load reversal occurs in the hydrostatic gear, i.e. in cases when the hydraulic motor assumes the function of a pump, as for example is the case when a load is lowered or when a constructional machine travels downhill, this control device does not operate satisfactorily. The cause of this is that on load reversal the working pressure in the working line concerned falls to a negligible level and hence the control piston of the piston-and-cylinder device moves the regulating valve in the sense of an increase in the intake volume of the main pump, which is now operating as a motor. The transmission ratio of the hydrostatic gear, and consequently also the braking effect of the driving motor, are thereby reduced. Thus the control device ceases to function and the driving motor begins to race.